Electroless copper plating is widely practiced in the electronics industry, particularly for plating through holes of printed circuit boards by the superior additive process. The current practice of electroless copper plating involves the use of formaldehyde as a reducing agent. Formaldehyde generally requires operation of the plating bath at a highly alkaline pH, greater than 11. The present plating bath operates at a pH less than 9, permitting electroless copper deposition on and in the presence of alkali sensitive substrates, such as polyimide and positive photoresist.
Prior plating baths utilizing Dimethylamine Borane (DMAB) as a reducing agent are known and described, for instance, in the article entitled "Electroless Copper Plating Using Dimethylamine Borane" by F. Pearlstein and R. F. Weightman, Plating, May, 1973, pages 474-476.
The same article refers to a plating bath containing both DMAB and Ethylenediamine Tetra-acetic Acid (EDTA) as a disodium salt with ammonium hydroxide for stabilizing the bath. The pH at room temperature was 10.7.
An electroless copper plating bath consisting of EDTA and triethanolamine with formaldehyde as a reducing agent, operating at a high pH level, is described in the book "Modern Electroplating" edited by F. A. Lowenheim, John Wiley & Sons, 1974, pages 734 to 739.
Additional prior art is found in U.S. Pat. No. 3,870,526 which refers to an electroless bath consisting of EDTA disodium salt and DMAB with ammonium hydroxide operating at a pH of approximately 10.7.
U.S. Pat. Nos. 4,273,804, 4,338,355, 4,339,476 refer to colloids and metallic dispersions which are seeds for further electroless plating.
U.S. Pat. No. 4,321,285 describes colloid based seeding and cobalt plating at pH in the range between 6 and 7 and copper plating with conventional formaldehyde based baths.
U.S. Pat. No. 4,318,940 describes stabilized colloidal dispersions for providing an economical process for preparing dielectric substrates for electroless plating.
Additional prior art is described in IBM Technical Disclosure Bulletin, Vol. 15, No. 1, June 1972 entitled "Autocatalytic Electroless Lead" concerning depositing of lead on copper; IBM Technical Disclosure Bulletin, Vol. 9, No. 10, March 1967 entitled "Chemical Nickel-Iron-Copper-Boron Films" concerning plating of alloys using tartate and ammonia complexes at high pH; and IBM Technical Disclosure Bulletin, Vol. 27, No. 1A, June 1984 entitled "Fast Plating Bath" concerning electroless copper plating using formaldehyde as the major reducing agent.